1. Field of the Invention
The invention refers to a mixing impeller including two engageable subassemblies, a method of manufacturing one of the subassembly and a method of assembling the two subassemblies.
2. Related Art
In the conventional engineering practice, a mixing device comprises a mixing vessel containing fluids to be mixed and a motor driving a shaft to which one or more mixing impellers are fixed. The mixing impeller may be mounted to the shaft by a hub which is bolted or welded in place so that it does not come loose during operation. The mixing impeller is typically specified along with the mixing vessel according to design guidelines (for example, Handbook of Industrial Mixing, Paul et al.) so as to achieve the desired performance. Unless there is a process change or a need for refurbishment the mixing impeller remains fixed to the shaft.
Some applications require that the mixing equipment is fully closed with no possibility of leakage between the mixing vessel and the environment—for example, the fluids to be mixed are either hazardous (e.g. toxic) or if they are sensitive to contamination from the outside environment (e.g. highly purified pharmaceutical material). In such cases a magnet drive system may be employed as a means of transmitting torque between an external motor and a mixing impeller inside of the mixing vessel. A driving magnet at the outside of the mixing vessel is driven by the external motor, and a follower magnet is arranged inside of the mixing impeller in the mixing vessel.
In contrast to the conventional mixing equipment, in which mixing vessels are typically fabricated from stainless steel or other alloys, single-use systems comprise plastic bags as mixing vessels and are used only once. Single-use systems are increasingly used in biopharmaceutical manufacturing operations because of the increased flexibility, lower capital cost, elimination of cleaning steps, reduced risk of cross-contamination, and reduced utility burden.
From the state of the art, single-use mixing impellers are known, which comprise a plastic impeller housing unitarily formed with a plurality of mixing blades extending from the impeller housing. One or more magnet(s) are inserted into cavities in the impeller housing, which is then closed by a plastic cover glued or ultrasonically welded to the impeller housing in order to prevent any fluid contact between the magnet(s) and the fluids to be mixed. The number of magnets and the geometry of each magnet both depend on the type of magnetic drive system being used. Mixing impellers as previously described are produced by e.g. Millipore or Pall Corporation (see e.g. LevMixer).
A range of mixing impeller geometries could be needed to meet performance requirements for a wide range of applications. For example, different diameters could be wanted to keep a constant D/T ratio (ratio of impeller diameter to mixing vessel diameter) so that the performance is consistent across different process scales. As another example, homogenization of shear-sensitive materials might require a low shear mixing impeller geometry, while a powder dissolution might require a high shear mixing impeller geometry.
Given the wide range of applications found in biopharmaceutical manufacturing, it is not likely that a single mixing impeller can meet all requirements. Two disadvantages become apparent as more mixing impellers are required. First, tooling must be created for each style of mixing impellers. This includes the molds as well as any assembly tooling. Second, an inventory of each mixing impeller variant must be held in order to ensure that the correct parts are on hand at the manufacturing facility where final assembly of the single-use mixing vessel is done.
Therefore, it is the underlying technical problem of the present invention to provide a mixing impeller which enables a wide range of applications in a cost-saving manner.